A number of known control arrangements regulate the position or elevation of implements, such as plows, attached to or drawn by agricultural vehicles, such as tractors. Such control systems generally sense the position of a three-point hitch and compare this position to a command or reference value set by an operator. Based upon this comparison, such control systems generate a control signal for an actuator to vertically move the hitch, along with the implement mounted on it, to the desired elevation.
Known control systems can also operate based upon the draft force generated by the interaction of an implement with the ground. Such draft force can be generated by the implement penetrating the ground, or by the implement being engaged with the ground. The control system typically compares the sensed draft force to a command or reference value set by an operator and generates a control signal for an actuator to vertically move the hitch to maintain the desired draft force.
Many systems for controlling implement elevation using implement position or draft force as an operating parameter are not adapted for controlling excessive wheel slippage (i.e., slip). Typically, a tractor experiences differing magnitudes of slip as the tractor encounters different soil conditions during travel over the length of a field. Although a certain level of slip may be desired for efficient operation, excessive slippage results in inefficient operation and, if severe enough, may result in the tractor's wheels burrowing into the field and becoming stuck. To maintain an efficient level of slip, yet avoid excessive wheel slippage, an operator is often required to change the position or draft command setting of the control system to accommodate variations in slip due to varying soil conditions. However, even an experienced operator may not respond quickly enough to soil conditions which cause excessive wheel slippage.
Control systems have been proposed to automatically respond to excessive wheel slippage by controlling the elevation of the implement. Generally, such control systems determine wheel slippage as the difference between the apparent speed and the true ground speed of the tractor. Upon detecting excessive slippage (i.e., when slippage exceeds a threshold), the control system reduces wheel slippage by raising the implement, thereby reducing the draft force on the tractor and increasing the transfer of implement weight onto the tractor to improve traction. For example, U.S. Pat. No. 4,518,044 discloses a system for controlling implement position as a function of various parameters including wheel slippage. The control system combines signals corresponding to wheel slippage, engine speed error and draft load command into a combined load error signal which may be used to control an actuator. The combined load error signal causes the actuator to raise or lower the implement, thereby reducing the combined load error signal.